1. Field of the Invention
The present invention relates to an apparatus for coating a liquid, such as a synthetic resin liquid or a paint on an object to be coated.
2. Description of Related Art
During production of a lithium iron battery, an electrode material, such as graphite, coke or carbon fibers is coated at a predetermined spacing onto a continuous metal foil such as an aluminum foil or copper foil. Furthermore, there are many occasions where a coating of a liquid such as a synthetic resin liquid or paint on a surface of a continuous sheet such as a film of a synthetic resin or a sheet of a paper is required. During an execution of these coating process, a length of the material to be coated is conveyed in a horizontal or a vertical direction by means of a roller member.
Furthermore, when a liquid such as a synthetic resin liquid or a paint is coated onto a surface of a cut length of an object to be coated, such as glass plate or wafer, the cut length of the object to be subjected to coating is conveyed horizontally by means of a belt conveyor or a roller conveyor.
When an object of a continuous length is to be subjected to coating, a slit die apparatus having a slit shaped outlet for the liquid extending along the width of the object to be coated is arranged at a location in front of a back up roller. A bath of the coating liquid is connected to a liquid delivery pipe, so that the liquid from the bath is fed to the slit die via a three way valve. The three way valve is also connected to are turn pipe, which allows excess liquid to be recirculated to the liquid bath.
In the operation the above mentioned coating system, a pump for generating a forced flow of the liquid is always operated, so that the liquid is always forced out. The three way switching valve is switched between an opened position where the liquid is discharged to the slit die and a closed position, where the discharge of the liquid is prevented.
The above system, where a discharge and stoppage of the liquid is done by switching of the three port switching valve between the opened position and the closed position, is defective in that the switching to the closed position to cause the liquid to be returned to the liquid reservoir does not instantly cause the feed of the liquid to be stopped, which results in an occurrence of a so-called a "post drop", which can cause a starting edge portion or finishing edge portion of the coating to be unsharp. Furthermore, when the liquid reservoir chamber is at atmospheric pressure, switching of the three way valve to a position where the liquid is returned to the reservoir causes the pressure to drop at the liquid feed pipe. Thus, after a subsequent switching of the three way valve to a position where the liquid is fed to the feed pipe, a stable discharge of the liquid from the slit die can not be obtained until the pressure of the liquid at the feed pipe is increased to a predetermined value, thereby causing the quality of the coating to be insufficient or poor.